fix: special case for static muted attribute in firefox

fix #6887

src/compiler/parser/index.js

@@ -556,6 +556,13 @@ function processAttrs (el) {
         }
       }
       addAttr(el, name, JSON.stringify(value))
+      // #6887 firefox doesn't update muted state if set via attribute
+      // even immediately after element creation
+      if (!el.component &&
+          platformMustUseProp(el.tag, el.attrsMap.type, name) &&
+          name === 'muted') {
+        addProp(el, name, 'true')
+      }
     }
   }
 }

test/ssr/ssr-string.spec.js

@@ -1000,6 +1000,15 @@ describe('SSR: renderToString', () => {
       done()
     })
   })
+
+  it('render muted properly', done => {
+    renderVmWithOptions({
+      template: '<video muted></video>'
+    }, result => {
+      expect(result).toContain('<video muted="muted" data-server-rendered="true"></video>')
+      done()
+    })
+  })
 })
 
 function renderVmWithOptions (options, cb) {

test/unit/modules/compiler/parser.spec.js

@@ -395,6 +395,15 @@ describe('parser', () => {
     expect(ast.props[0].value).toBe('msg')
   })
 
+  // #6887
+  it('special case static attribute that must be props', () => {
+    const ast = parse('<video muted></video>', baseOptions)
+    expect(ast.attrs[0].name).toBe('muted')
+    expect(ast.attrs[0].value).toBe('""')
+    expect(ast.props[0].name).toBe('muted')
+    expect(ast.props[0].value).toBe('true')
+  })
+
   it('attribute with v-on', () => {
     const ast = parse('<input type="text" name="field1" :value="msg" @input="onInput">', baseOptions)
     expect(ast.events.input.value).toBe('onInput')